link-state: virsh: Add wrapper commands for changing link state

link-state: virsh: Add wrapper commands for changing link state

Two new commands are added to virsh that wrap usage of
virDomainUpdateDeviceFlags for changing link state of domain's network
interfaces. These wrappers extract network devices's xml configuration
and modify the link state for easy manipulation from an user's perspective.

 - domif-setlink - set link state of a domains virtual network interface
 - domif-getlink - get link state

* tools/virsh.c   - Add functionality to virsh
* tools/virsh.pod - Manpage documentation